The dominant mode of signal propagation in some known communications systems that have relatively large distances between the receiving and transmitting antennas is electromagnetic fields. Due to their ability to travel relatively long distances, noise attributed to electromagnetic fields can be a significant problem. Known communication antenna designs attempt to optimize the pickup of electromagnetic, electric and magnetic fields, but few attempts are made to reduce the affects of propagated and conducted electromagnetic noise, propagated and conducted electric field noise, and propagated and conducted magnetic field noise.
For short-range communication applications, the antenna must have good near field performance and should reject unwanted signals and/or noise from unwanted radiating and conducting sources. Examples of such short-range communication applications include, but are not limited to, hearing aid to telephone receiver communications, hearing aid to hearing aid communications, and programmer to hearing aid communications. Examples of unwanted radiating sources include radio and television stations, and the like. Examples of unintentional radiating sources of unwanted electromagnetic interference (EMI) include computers, televisions, electric motors and the like. Examples of conducted interference sources are electric field and electromagnetic radiation conducted via electrically conductive objects such as metal, human skin and conductive liquids significantly impairing communications.
There are other design challenges facing short range communications systems. Virtually all short-range communication systems are compact, battery or RF powered and low-cost. Because of the previous requirements, most all short range communication systems are half-duplex or simplex communication systems that are capable of transmitting data in only one direction at a time. Known half-duplex or simplex communications system use electrical or mechanical switches to change between receive and transmit modes. These switches require a significant amount of time to switch between modes. Other problems include the additional parts and separate control lines to select between the receive and transmit modes.
The communication system is tuned to transmit and receive communication signals at a desired resonant frequency. However, the equivalent parallel parasitic capacitance of the coil and the capacitance of the low noise amplifier and other portions of the circuit can detrimentally and unpredictably affect the resonant frequency of the communication system.
There is a need in the art to provide improved communication systems and methods for transmitting and receiving short range data.